1. Field of the Invention
The present invention relates to a method of carrying out operations in underground/subsea oil/gas wells, preferably by the utilisation of coiled tubing to carry the work tool. More specifically, this method is meant to be used for advancing a rotating downhole tool in an underwater well, wherein said tool is brought to rotate by means of a downhole motor carried by the coiled tubing. Thereby, the method is of the kind specified in the introduction of claim 1.
2. Description of the Related Art
Also, the invention relates to an apparatus of the kind, which may be employed to implement or support the effect of the method according to the invention, and which comprises a motorized downhole tool, which is arranged to be connected to a pipe string/rod string, preferably coiled tubing, and to receive the torque for the rotation of the tool from the motor. The apparatus according to the invention is thereby of the kind appearing in further detail from the introductory part of the following first independent claim to the apparatus.
Also, the invention comprises a particular application of the method/apparatus.
When the exploitation of a sea-based oil/gas field is considered no longer financially profitable, and the underwater wells are about to be shut down and abandoned, the wells are to be plugged in a reliable manner.
To ensure proper plugging of each of the underwater wells by grouting, the inner casing (run last) must be withdrawn, so that cement mixture can be filled all the way out to the wall of the well. It is not sufficient to fill cement mixture into the inner casing, because formation fluid penetrating into the annulus, could penetrate further up and out of the well if the cement mixture, which has surrounded the casings already from the cementing thereof, is not tight.
To withdraw the (inner) casing, break it up and. transport it to shore is very laborious. Therefore, the oil companies are interested to find a solution, whereby the casing will remain in situ, while at the same time, the well is plugged in accordance with regulations.
This can be achieved by running a cutting tool into the well, cutting away the inner casing in an area below the other casings. A rotating cutting tool is lowered into the casing to the desired depth, where the pivotal blades of the tool are folded out gradually, cutting the casing. Then the tool is displaced in the well while it is rotating and milling and drilling out the casing from the end at the cutting point. When about 15 metres of the casing wall has been drilled out and milled away, the operation is completed, and the equipment can be pulled up. Then, when cement mixture is filled into the inner casing, the cement mixture can penetrate all the way out to the formation in the area from which the casing has been milled away.
Several solutions for milling/drilling tools have been suggested (milling tools, grinding or chipping tools, normally arranged to be mounted in the place of the drill bit).
Since, in general, there are no drill rigs on the platforms normally employed for the implementation of the operations relevant in connection with plugging of underwater wells, which are to be abandoned, it is desirable to be able to use coiled tubing to enter the well with tools. The alternative is to mount a drill rig on the platform, but that is both expensive and time-consuming.
However, coiled tubing will not be able to absorb sufficient torque from the cutting/,milling/drilling tool like an ordinary drill string could have done, and thus it is imperative to have extra torque-absorbing equipment mounted in association with the coiled tubing.
In the technical field of the present invention the insufficient capacity of coiled tubing to absorb torques is considered a qualified problem in connection with motorised rotating downhole tools.
A previously known suggestion, which oil companies have found interesting, involves anchoring a hydraulic piston-and-cylinder, with a piston travel of a couple of metres, at the end of the coiled tubing, and securing an assembly comprising tools with a motor arranged thereto, to the end of the piston rod of the piston-and-cylinder.
In the execution of said downhole operation by means of the rotating motorised tool, a hydraulically expanding clamping ring (or other expanding clamping device) provides for fixing the piston-and-cylinder in the casing and absorbing the torque from the driven rotating tool, while the piston-and-cylinder causes advancing of the tool.
When the piston-and-cylinder has advanced the tool a distance corresponding to a length of stroke, the expanding clamping ring is released, and the apparatus (downhole tool+driving motor) is moved forward a distance corresponding approximately to a length of stroke in the direction of advancing. The clamp ring is tightened again, and the tool is displaced to the milled end of the casing, and the process is repeated.
However, an ordinary hydraulic piston-and-cylinder, in which the piston and piston rod have circular cross-sections, cannot absorb any torque. Therefore, also in this known device extra measures are necessary to handle the torques, such as formation of longitudinal grooves in the piston rod and the slip at the end gable of the cylinder, or so-called splines (grooves, flutes etc.), a particular guide rail or other means can be used. This complicates the equipment and it will all be very expensive.
In accordance with the present invention it has been established, among other things, that apart from its inability to absorb torques, coiled tubing exhibits considerable strength properties and is more than strong enough to endure the advancing force proper.
Thereby the general object of the invention has been to reach and prescribe a method of the kind specified in the introductory part of claim 1, whereby, based on simple operational steps, the drawbacks described in the preceding are remedied, and whereby also in other respects, a technique advantageous in terms of work and time and also economy, is obtained.
According to the invention the object has been realised through a procedure as specified in the characterising part of claim 1.
The operational steps utilised by the method in order to reach said aim, consist essentially of connecting the downhole motor to a carriage which is arranged partially to drive inside a casing in the well, which is to be plugged, partially to absorb the torque of the downhole motor utilised by the rotatable tool (cutting tool); connecting the carriage to the coiled tubing (or other string not absorbing torques) by a swivel connection in order to avoid transmission of torque from carriage to coiled tubing, and pulling the coiled tubing in order to supply an advancing force to the downhole tool.
The upward advancing represents a simplified method of advancing the downhole cutting tool, and is effected through an upward pull on the coiled tubing. The advancing force that the coiled tubing is thereby subjected to, hardly constitutes more than about five percent of the tension allowed in the coiled tubing. Thus, the coiled tubing is more than strong enough to endure and withstand this advancing force; it is the torques that are problematic by coiled tubing, and the swivel coupling solves this problem in a simple manner. These features in combination provide a technical effect considered to be fairly important within the art in question.
The apparatus according to the invention comprises the above-mentioned particular carriage, which is equipped with driving wheels arranged to be forced radially outwards into bearing abutment on the inner casing wall and thereby absorb the torque through friction.
The wheels are directed along the well, so that the carriage can be displaced along it while the wheels are forced against the inner wall of the casing.
As mentioned in connection with the method according to the invention, the carriage will be connected in use to the coiled tubing by a swivel coupling, so that the carriage can rotate relative to the coiled tubing if the wheel should lose their grip. It is important to prevent the torque from the rotating tool from being la transferred to the coiled tubing, and twisting it about its longitudinal axis, if this should happen.
In use the rotatable shearing/cutting/drilling/milling tool with the associated driving motor is lowered by means of coiled tubing or a similar string to the desired depth in the well, and the wheels of the carriage, which is of a kind described as a xe2x80x9crolling anchorxe2x80x9d, are forced outwards against the inner casing wall. Each wheel has a radial cylinder arranged thereto, to which pressure fluid is supplied. Pressure in the fluid circulated through the coiled tubing to drive the motor rotating the cutting/milling tool, may be utilised in a known manner to force the carriage/ anchor wheels radially outwards into bearing abutment on the internal wall of the casing. Separate hydraulic pressure fluid (hydraulic oil) may alternatively be supplied through a separate hydraulic line, which runs inside the coiled tubing in a known manner.
The cutting tool first cuts through the casing wall, from inside radially outwards, by shears being folded out (e.g. hydraulically). Then the cutting tool is advanced upwards by the coiled tubing being pulled. Thereby the carriage absorbs the torque from the tool, while the advancing force is being supplied from the coiled tubing.
When coiled tubing is used for the advancing of the downhole tool, and, as mentioned, this is preferred, it is also worth noticing that a condition of this is that the tool is advanced upwards through a pull on the coiled tubing. The coiled tubing cannot provide any particular downward force. However, this upward advancing is not at all disadvantageous for the cutting/milling/drilling work, which is to be carried out by the motorised rotating downhole tool.
In the following there will be described a non-limiting example of a now preferred embodiment of an apparatus for use in the execution of operations in a well, especially in connection with work tools connected indirectly to coiled tubing in order to be advanced (normally upwards) by means thereof. The method according to the invention followed in the advancing of the rotating downhole tool, will appear, at least implicitly, from the description of the constructional configuration and function which distinguish the apparatus, which can be concretised in many different ways within the scope of the present invention which has been set out in the following claims. The term xe2x80x9crolling anchorxe2x80x9d is used more or less to associate the carriage to the prevalent term for such drivable devices provided with wheels, relying on friction.